I think part of the problem is that a lot of people (myself included) use tablets for decidedly "non-pro" stuff that doesn't require much cpu power, like web browsing, reading, looking at pictures, reading emails etc.
The iPad Air & Air 2 still work well enough for these people and the iPad Pro prices (629€ for 32 GB, 769€ for 128 GB here in Germany) make upgrading rather unappealing.
IMO, the "Pro" focus is detrimental for the iPad's broad market appeal. From what I can see, the number of people who actually replace their notebooks with iPads is practically non-existant, and it is priced too high for the companion device status that it has for most.
Also, here's my completely non-scientific market research on Amazon.de:
iPad Air 2 64 GB: sales rank #22 in tablet pcs
iPad Pro 9.7 32 GB: rank #239 in tablet pcs
iPad Pro 9.7 128 GB: rank #962 in tablet pcs
Ouch.